This invention is directed to removing and preferably recovering silver from solutions such as used photographic fixer solutions.
Dissolved silver is commonly removed from solutions to recover the silver as a valuable product and/or to remove the silver as a pollutant. For example, silver is removed from photographic solutions to recover the silver as a valuable product as well as to permit the solution to be discharged without causing pollution.
Numerous devices have been developed for removing silver from used photographic fixer solutions. Such devices are described in U.S. Pat. Nos. 3,369,801; 3,692,291; 4,035,181; and 4,156,604; all of which are incorporated herein by this reference.
These devices utilize an electron exchange reaction, i.e., an electrochemical exchange reaction which occurs between a more noble metal and a less noble metal in order to recover the silver in the used solution. The less noble metal in these devices is iron in the form of inexpensive steel wool or a window screen. When the silver rich solution is passed over the iron, an electron exchange occurs wherein the iron metal Fe.sup.0 is oxidized to ferric or ferrous ion and the silver ion (Ag.sup.+) is reduced to silver metal (Ag.sup.0). The silver precipitates from the solution to form a sludge, from which the silver is recovered. The spent solution from the device is discharged to a sewer or returned to a fixing tank.
The use of steel wool or window screen as the silver source has significant disadvantages. Among these problems is inefficient use of the iron, a substantial portion of the iron not being used to replace silver. Further, frequent clogging and channeling have been experienced. Channeling is a particular problem as the iron in the steel wool or window screen is depleted, resulting in voids forming in the exchange medium. These voids provide a path of lower resistance to flow and loss of efficiency in silver recovery and material use.
A further disadvantage is that these units can require the use of steel wool or window screen of different sizes or weights dependent upon the silver content or pH of the solution being treated. This can require the availability of two or more types of silver recovery devices, or the use of a plurality of devices.
Another disadvantage of existing recovery devices is that it is difficult to purify the recovered silver. The recovered silver is mixed in with large particles of window screen or steel wool. In order to refine the silver, it is first necessary to comminute the silver sludge containing particulate iron into a fine powder, such as by ball milling. This is a time-consuming and costly step.
A further disadvantage of the existing devices is that leakage between the container lid and body can occur. A monocoque container cannot be used because the steel wool or window screen needs to be placed in the container. Such leakage can cause significant property damage.
In view of these problems, it is apparent that there is a need for a silver recovery apparatus that makes efficient use of the exchange medium, avoids clogging and channeling in use, does not leak, maintains good contact with the silver-containing solution, can be used for a wide variety of solutions having different silver contents and different pH, and recovers silver in a form that does not require comminution for refining.